The high speeds generated by helicopter tail rotors present hazards to helicopter crews, passengers, and bystanders during routine landing and takeoff, and especially in the course of high-risk applications such as military operations, search and rescue, and emergency medical services. A means for adjusting the speed of a helicopter tail rotor independent of a primary rotor would be beneficial in that the tail rotor could be stopped or reduced to safe speeds when the helicopter is not in flight but while the primary rotor is still engaged. However, countervailing design considerations have resulted in few, if any, helicopters capable of independent adjustment of the tail rotor speed. Thus, there is a continuing need to find new ways by which the rotational speeds of aircraft rotors, propellers, and proprotors can be reduced.
The high speeds of helicopter tail rotors, which may rise to nearly super-sonic levels, are also responsible for producing excess noise. Rotor noise presents a nuisance to the general public in terms of noise pollution as well as a strategic disadvantage during military operations in that the element of surprise may be prematurely compromised, jeopardizing aircraft personnel and mission objectives. Thus, the aviation industry continually seeks new ways by which the rotor noise of rotor-propelled aircraft can be reduced.
Noise issues are further complicated in that the sound produced by helicopter tail rotors changes with altitude because the air thickness decreases and distance from a listener increases at greater elevations. Thus, a need exists to find new ways by which noise from rotary aircraft can be dynamically controlled.